A critical process in the manufacture of single wafers of silicon today is that of plasma or reactive ion etching in which, for example, a layer of silicon dioxide is removed from the surface of the wafer in regions which are not protected by resist. The etch process is undertaken in the presence of fluorine-containing gaseous species, and involves removal of silica as volatile SiF4. During the process the wafer must be held in a tightly controlled location in a jig, which must impart no impurities to the wafer. Such jigs are usually made from quartz glass. Typical sizes for the blanks, from which quartz fabricators machine the final parts, are 420×353 mm, 418×334 mm and 442×365 mm. Such blanks are conveniently cut from large diameter hollow ingots, and for economic reasons the quartz glass is generally made by fusion of natural quartz grain. Potential drawbacks with the use of natural quartz, however, are that it typically contains metallic impurities which can be transferred to the wafer and that the glass may contain certain defects, such as microbubbles and inclusions.
The conditions for removing the oxide layer from the wafer are such that some etching of the quartz glass jig also takes place, so that its dimensions change progressively, and the life of such a jig is limited thereby. Furthermore, such etching can expose any microbubbles and inclusions, and this leads to the release of particles which can be a serious cause of defects in the product. The concentration of such microbubbles and inclusions depends on the method of manufacture of the quartz. Both defects are more numerous in quartz glass derived from electrically fused boule than in flame-fused quartz; indeed it is typical that manufacturers ignore bubbles and inclusions of size less than 80 or 100 μm. High quality semiconductor jigs are therefore generally made from flame-fused quartz, derived from natural quartz crystal. However, even the best quality glass made from natural quartz will contain impurities. Typical metallic impurities are all greater than 100 ppB by weight, and some may be at a level of several hundred ppB, which can lead to the release of particles during the etch process. In this specification, a distinction is made between impurities (e.g. metallic contaminants), which may have a deleterious effect on the properties of the glass or on the process in which it is to be used, and dopants, which may be metallic or non-metallic, and may have a beneficial effect on the product or on the process.
Small bubbles and inclusions are a feature of the quartz glass products which have been accepted in the past for the manufacture of jigs in the semiconductor industry. Bubbles and inclusions are present in relatively large numbers when the glass is made from natural raw materials, and an industry specification for a fused quartz glass may quantify such defects in terms of their overall cross sectional area (CSA) in a given volume of glass, and/or in terms of number (above a certain minimal defect size) which can be counted in a representative volume of the glass.
When very small, such bubbles and inclusions are difficult both to detect and to quantify, and typically, when their size has been less than e.g. 80 μm or 100 μm, they have not been counted in specifications. It may furthermore be difficult to distinguish between a small bubble and a small inclusion, so that the two defect types are frequently combined within a specification and described in some cases as “bubbles”, and in others as “inclusions”.
A typical electrically fused quartz glass sold for the manufacture of semiconductor jigs has such a specification, which notes the total CSA of all the bubbles in a representative volume of glass, and also the actual number of bubbles in such a representative volume, and also notes the minimum size of bubble included in the count, thus:
Total CSA of BubblesMax. No. of Bubblesin 100 cm3 (mm2)in 100 cm31.515000Bubbles less than 0.08 mm diameter not counted
On the other hand, a typical flame fused quartz glass sold for similar applications has the following specification:
Max. No. of Bubbles/InclusionsSize (mm)in 100 cm3>0.500.1-0.5≦50Bubbles/Inclusions less than 0.1 mm diameter not counted
Synthetic silica glasses have in the past been sold generally for optical applications, and have therefore been of higher visual quality. Today, the specification is typically in terms of CSA of all bubbles and inclusions present in a representative volume, e.g. 100 cm3 (as has been described in specification DIN 58927). Thus, the available grades of a typical synthetic silica product may be specified as follows:
Total InclusionMaximumCross SectionSizeClassin 100 cm3 (mm2)(mm)0≦0.030.101≦0.10.282≦0.250.503≦0.50.764≦11.005≦2.01.27
The present invention is primarily concerned with high quality components for the semiconductor industry, with bubble and inclusion content of a quality generally comparable with Class 0.
It might appear obvious to seek to replace the natural fused quartz jig with one made from synthetic vitreous silica, which could be of higher purity, and substantially free from microbubbles and inclusions. However, this solution has not been followed generally, partly because of the complexity of manufacture of these large diameter products from available synthetic silica ingots, leading to unacceptable cost, and partly because of the relatively high etch rate of the available synthetic vitreous silica glasses. Thus the largest synthetic vitreous silica ingots have to date been made by the “direct process”, i.e. deposition of glass directly from the silica synthesis flame from one or more burners. In this way boules of diameter up to 2 meters may be made, but manufacture of large annular parts from such boules can only be achieved with significant wastage of unused material. The fact that the required rings have rather low ratio, i.e. outside diameter/inside diameter, makes matters worse. For example, the ratio of the blanks mentioned above is 1.19, 1.25 and 1.21 respectively. Manufacture of such low ratio rings from the solid leads to major losses of material, which may be unusable for other applications. Furthermore, the direct process yields glass with typical OH content 600-1,200 ppM (parts per million), which has the effect of reducing viscosity and increasing the etch rate under typical plasma etch conditions. This is a further reason why these processes are not used in the manufacture of semiconductor jigs for these applications.
Low-OH synthetic silica rings would appear to be better suited to these applications, but such low-OH glasses are achieved by two stage processes. Typically, silica soot is deposited from a synthesis flame to form a porous soot body, which may be subsequently dehydrated (typically by heating in an atmosphere of chlorine) before sintering to pore-free glass either in an atmosphere of helium or under vacuum. The main deposition processes used for manufacture of such glasses are VAD (Vapour-phase Axial Deposition) to make a solid cylindrical soot body, subsequently sintered to form a solid cylinder, and OVD (Outside Vapour Deposition), which involves deposition of silica soot on a mandrel, which is subsequently removed, and the soot body is subsequently sintered to a tubular body. VAD sintered bodies are generally of rather small size for use in semiconductor jigs, and would require extensive and costly reprocessing to achieve the required hollow cylindrical products. Such reshaping of the glass would also introduce severe risk of contamination of the surfaces of the glass body in contact with any graphite tools etc., and that may require subsequent removal of the external surfaces by machining, by etching with acid, or by both techniques. Hitherto, OVD technology has been used for the manufacture of optical fibre materials, and the largest diameter ingots made in this way have been typically of size 200-250 mm diameter, have generally been of heavy ratio, and have been contaminated with chlorine, which is known to increase the etch rate of quartz glass. Even if the purity had been acceptable, reprocessing of such ingots to give the required large diameter low ratio rings would be uneconomic, and would again risk contamination, requiring removal of the surface layers of the product.
As an alternative to the use of synthetic silica for semiconductor jigs, efforts have been made to increase the etch resistance of quartz glass made from natural quartz crystal. Limited success has been achieved by doping the quartz glass with the oxide of one or more metals which have fluorides of volatility less than that of silicon. Thus, doping with aluminium oxide, optionally mixed with the oxide of one or more rare earths, has been proposed as a means to reduce the etch rate of the quartz glass. See, for example, U.S. Pat. No. 6,887,576, U.S. Pat. No. 7,365,037, U.S. Pat. No. 7,661,277 and U.S. Pat. No. 7,084,084.
This approach may lead to an improved etch resistance; however it suffers from the potential disadvantage that, when some etching of the surface of the doped quartz glass does occur, it exposes islands of the dopant oxides. This leads to undesirable roughening of the surface, and ultimately to the release of micro-particles of the dopant oxide, which can cause defects in the wafer.
An alternative technique has therefore been explored, in which efforts have been made to incorporate nitrogen in the glass (see US 2008/0066497), optionally in the presence of additional dopant metals (see US 2008/0053151 and US 2009/0163344). Limited concentrations of nitrogen have been achieved either in the surface of a quartz glass article, or alternatively in the bulk, by heating the product or an intermediate in gaseous ammonia. However, when significant quantities of nitrogen are present, there is a danger of degassing or bubble formation during any further hot working of the glass. It would evidently be preferable if the glass could be manufactured in a near net shape form that did not require further hot working to achieve the dimensions of the final product.
Increase in viscosity of synthetic vitreous silica has also been reported following doping with carbon, or with both carbon and nitrogen in combination. This may be achieved by heating the porous soot body in an atmosphere of, for example, the vapour of a siloxane, a silazane, or other organic species, optionally in the presence of ammonia (see US 2006/0059948).
The effect of doping with carbon or nitrogen as a means to reduce the etch rate in a plasma etch environment is thought to be associated with the increased viscosity induced by incorporation of these species in the lattice. It has furthermore been found that the rate of etching is increased if the glass contains significant amounts of certain non-metallic contaminants, notably OH (hydroxyl), chlorine, and fluorine. These are species known to cause disruption to the network of bonds in the silica structure. It has also been shown that there exists a negative correlation between the viscosity of the glass and the etch rate, and since the viscosity of the glass increases with decrease in fictive temperature, so it might be expected that a reduced etch rate should result from careful annealing of the glass to achieve low fictive temperature.
Thus, the glass required for an etch-resistant semiconductor jig is required to contain a minimal content of OH and a minimal (preferably zero) level of chlorine. Preferably, it is substantially free of fluorine, and it may be optionally doped with a low level of nitrogen, carbon, or possibly a combination.
While low-OH content glass can be achieved by electric fusion of quartz crystal powders under reduced pressure, such products generally contain significant levels of bubbles and inclusions. Where synthetic vitreous silica products are manufactured, these are typically made by vapour deposition from a flame, usually fed with silicon tetrachloride as precursor, to form a porous silica “soot body”. Such soot bodies are generally dehydrated by heating in an atmosphere containing chlorine, and subsequently consolidated to pore-free glass by sintering under reduced pressure, or in an atmosphere of helium. The vitrified products retain significant levels of chlorine, which is difficult to remove unless strenuous efforts are made by additional processing.